Now that I’ve read (most of) it, it’s worth noting that the Ecto book also has a chapter on application design (covers pure/impure, Contexts, Umbrellas) while of course Programming Phoenix also covers application design in terms of Contexts - I highly recommend both books 
Other books not mentioned here that cover app design (that I haven’t read yet) are Functional Web Development with Elixir, OTP, and Phoenix (Pragprog) and Designing Elixir Systems with OTP (Pragprog) and The Elixir Toy Robot (self-published) may also be worth a look
